1. Field of the Invention
The present invention relates to an aperture grill and, more specifically to an aperture grill provided with a plurality of parallel, vertical slits arranged at regular intervals, to be used on a Trinitron picture tube.
2. Description of the Prior Art
In a Trinitron picture tube as shown in FIG. 5, three parallel electron beams 2 emitted by cathodes 1 are converged on the center axis of the Trinitron picture tube by a prefocusing electron lens 3 and a main focusing electron lens 4. The three electron beams 2 then diverge, the diverged three electron beams 2 converge on an aperture grill 6, namely, a shadow mask provided with a plurality of parallel, vertical slits in the effective area of the picture, by a convergence deflector 5. Then the three electron beams 2 pass through the slits of the aperture grill 6 and strike their intended color phosphor dots arranged to form stripes 7, respectively, among those formed on the inner surface of the panel of the Trinitron picture tube to make the intended color phosphor dots luminous.
The aperture grill 6 of the Trinitron picture tube has the function of a color screening electrode. Generally, the aperture grill 6 is constructed by attaching a flat grill plate, namely, a thin mild steel plate provided with a plurality of parallel, vertical slits (apertures) formed at regular intervals by etching in an effective area of picture, to a steel frame in a properly taut state.
In assembling the aperture grill 6, the flat grill plate is attached by seam welding to the frame elastically buckled by pressure, and then the pressure is removed from the frame to allow the frame to restore its normal shape by its own elasticity so that the flat grill plate welded to the frame is held in a taut state. Since tension distribution in the flat grill plate is not satisfactorily uniform when the frame is allowed to restore its normal shape by its own elasticity after welding the flat grill plate thereto, the respective widths of the slits of the flat grill plate are liable to differ more or less form each other.
The characteristics of the aperture grill, particularly, the curvature of the aperture grill corresponding to that of the panel, and the tension of the flat grill plate and the like necessary for the quality control of the aperture grill, are measured. The widths of the end slits at the opposite ends of the arrangement of the slits are inspected visually. This visually, which, however, is not necessarily a perfectly effective means of inspection.
Some aperture grills for a superfine picture tube are provided with dummy slits. The dummy slits are used for adjusting the widths of the end slits. In welding the flat grill plate to the frame, the dummy slits absorb irregularity in the distribution of tension in the flat grill plate so that the respective widths of the slits are equal to each other. However, it is not appropriate to use such an aperture grill provided with dummy slits as a mask in an exposure process for forming a fluorescent layer on the surface of a panel; portions of the panel outside the effective area of the picture corresponding to the dummy slits are exposed. Accordingly, when the aperture grill provided with the dummy slits is used as a mask, the dummy slits must be covered with masking means and the masking means needs to be removed after the exposure process, which requires an additional process. Thus, the use of the aperture grill provided with the dummy slits for the exposure process requires additional labor and additional equipment, and it is possible that defects are introduced into the aperture grill in covering the dummy slits. Accordingly, the use of the aperture grill provided with the dummy slits is not desirable.